1. Field of the Invention
The invention relates to a method for producing an aqueous electrode paste, which includes thickly kneading a powder including an active material and a thickener and an aqueous solvent to form a mixture and diluting the mixture with an aqueous solvent to produce an aqueous electrode paste, and a two-shaft kneader.
2. Description of Related Art
As a power generating element of a battery, an electrode body that is obtained by laminating or winding a positive electrode, a negative electrode, and a separator and is impregnated with an electrolytic solution is used. The positive electrode and negative electrode are formed by forming coated films by coating electrode pastes for positive electrode and negative electrode on a positive electrode current collector and a negative electrode current collector, drying, and pressing the coated films.
The electrode paste is produced in such a manner that an active material, a thickener, and a solvent are thickly kneaded to form a mixture, the mixture is diluted with a solvent, and thereafter a binder is added to the diluted mixture. Thick kneading, dilution, and addition of a binder are performed using a kneader such as a planetary mixer, for example. A kneading machine kneads an active material, a thickener, a solvent and a binder that are charged en bloc respectively during thick kneading, diluting, and addition of the binder by rotation of a stirring blade. In this case, the electrode paste attached to the stirring blade has to be scraped off with a paddle during kneading. Therefore, the working hours increase, thus increasing the direct labor cost.
According to a technology disclosed in Japanese Patent Application Publication No. 2005-222772 (JP 2005-222772 A), an electrode paste is produced using a continuous two-shaft kneader in which to two rotary shafts rotatably supported by a hollow barrel, screws and paddles are provided. According to the technology disclosed in Japanese Patent Application Publication No. 2005-222772 (JP 2005-222772 A), a powder (active material and thickener) and a solvent charged inside of the barrel are conveyed with a screw, and thickly kneaded by rotation of the paddle. Thereafter, according to the technology disclosed in Japanese Patent Application Publication No. 2005-222772 (JP 2005-222772 A), a mixture of a powder and a solvent is diluted with a solvent further charged inside of the barrel to produce an electrode paste.
As the electrode paste, from the viewpoint of material cost and reduction of waste that is generated during production, an aqueous electrode paste that uses an aqueous solvent such as ion-exchange water is used. In this case, as the thickener, for example, CMC (carboxymethylcellulose) is used. The thickener except a part thereof absorbs the aqueous solvent during thick kneading and swells. That is, a part of the thickener absorbs an aqueous solvent during dilution to swell.
In the technology disclosed in Japanese Patent Application Publication No. 2005-222772 (JP 2005-222772 A), in what state the aqueous solvent is charged is not specifically disclosed. In general, it is considered to charge a rod-like (that is, a state that is not microparticulated) aqueous solvent through an orifice plate. When such a rod-like aqueous solvent was charged as a solvent used during dilution, insoluble cellulose contained in a part of the thickener rapidly swells and may generate microgel having a large size. As a result thereof, the microgel may not be sufficiently pulverized by rotation of the barrel.
That is, in the above example, when the aqueous electrode paste is produced, many microgels larger than a thickness of a coated film remain in the aqueous electrode paste. Therefore, in the coated film, many coating defects such as lack of hiding where the coated film is not present and pin holes are generated. This is the same also when an aqueous solvent is charged en bloc in a kneader such as the planetary mixer. Therefore, when the aqueous electrode paste is produced according to the example, a yield of batteries is degraded.
Further, when the aqueous electrode paste is produced according to the example, the maximum size of the coating defects becomes larger. Therefore, due to lithium precipitation, an increase in the battery resistance may occur.